Bring error unpipe overrides into core

• Document this option

This will be off by default, but I think this is a common enough case that people would like this.

I envision the API looking like this:

gulp.src(['**/*.js'], {partial: true})

will block unpipe events downstream

[phated]

Node’s recommended pattern is to use streams.pipeline() now - We’ll be documenting this as “Getting Started - Error Management” so I’m going to close this.

[phated]

@contra The complex solution would be for gulp.src to return a mock stream that exposed a pipe method that queued streams into pump or pumpify but that seems overkill and makes gulp.src streams behave differently than browserify or other node streams.